There is a growing interest in uncooled, infrared (“IR”) imaging for both military and commercial applications such as night vision and heat sensing for fire alarms. Modern silicon micromachining, combined with advances in read-out circuit design/fabrication and digital signal processing, have resulted in uncooled, IR focal plane arrays becoming commonplace in many imaging applications.
Resistive bolometers, which are well known in the art, are particularly attractive for uncooled thermal detector applications, such as an uncooled focal plane array (UFPA), because they have relatively high responsivity and can be fabricated with relatively greater ease than other types of detectors, such as thermopile detectors and pyroelectic detectors.
In a focal plane array device, it is desired that the bolometer material fulfills the following criteria: 1) adequate resistivity to match the read-out electronics; 2) good ohmic contact with low contact resistance between the bolometer and the leg metal; 3) high temperature coefficient of resistance (TCR), preferably exceeding 2%; 4) low 1/f-noise; 5) ability to be deposited using a technique compatible with existing microbolometer fabrication processes; and 6) stable electrical properties.
Conventional uncooled bolometers include detectors based on vanadium oxide VOx, amorphous silicon Si and semiconducting YBCO compound. These materials may fulfill most of the above criteria, but typically do not have stable electrical properties to satisfy the sixth criterion.
Currently, VOx is the microbolometer material most commonly used in an uncooled IR camera because of its high Temperature Coefficient of Resistance (TCR), adequate resistance and low 1/f-noise values. However, the resistance of the VOx bolometers is unstable under certain operating conditions. For example, exposing microbolometers (pixels) manufactured from VOx to Joule heating or infrared heating results in the resistance of the microbolometers varying with a long decay time to equilibrium and, for certain conditions, a permanent residual resistance change. It causes pixel-to-pixel variations in an UFPA. Depending upon the extent of the resistance aging induced instability, the instability may be automatically electronically compensated to maintain high performance of the VOx UFPAs. In many cases, however, resistance aging effects that result from the infrared heating associated with the operational condition of pixel heating cannot be automatically compensated. These aging or “memory” effects are seen in thermal images generated by cameras, which is undesirable. In addition, resistance instability of VOx often leads to a loss of yield and an increase in the cost of VOx UFPA production.
U.S. Pat. No. 5,698,852 discloses a bolometer for uncooled IR detectors in which the bolometer is made of pure titanium or a titanium alloy (e.g., titanium combined with another metal) in order to have a specific resistance near 47 μΩ-cm and a resistance change (or TCR) of 0.3% for a lower 1/f noise ratio than conventional VOx bolometers. However, using a bolometer made of pure titanium or a titanium alloy typically requires the resulting resistor to be very long and serpentine in shape to have a resistance compatible with readout electronics wiring while maintaining a TCR of 0.3% in comparison to other conventional bolometers (e.g., a VOx bolometer), resulting in the titanium bolometer occupying a significant amount of space on the substrate of an uncooled focal point array for an IR detector.
A few patent references (US Patent App. No. 2003/0209668 and U.S. Pat. Nos. 6,198,099; 6,144,030; Re. 36,706; 5,629,521, 5,584,117, and 5,010,251) have identified titanium oxide (TiOx) as an alternative to VOx as the resistive material layer for a bolometer. However, each of these references, fails to disclose a structure type (e.g., crystalline, polycrystalline, or amorphous) or composition for the titanium oxide layer, each of which can significantly impact properties of the resulting bolometer. Moreover, several of these references (US Patent App. No. 2003/0209668 and U.S. Pat. Nos. 6,198,099; 6,144,030; Re.36,706) and one other reference (U.S. Pat. No. 6,489,613) that mentions Ti2O3 as a potential composition of a bolometric material, each teach using a VOx structure and composition as the preferred material for a bolometer despite the aging or “memory” effect problems associated with the resistance instability of VOx.
Therefore, there is a need for a resistor material that overcomes the problems noted above and others previously experienced for bolometers. In particular, there is a need for a bolometer resistor material that has a high TCR, low 1/f-noise, provides for higher yield UFPA production at lower cost, and has substantially stable resistance that avoids the above identified memory effect problem.